Such contactless fill level measuring devices working according to the pulse radar principle are applied in a large number of branches of industry, especially in the processing industry, in chemistry and in the foods industry.
Conventional pulse radar-fill level measuring devices have regularly a transmission system having a pulse producing system connected to a control unit, which is embodied in such a manner that it produces for each measurement a transmission signal composed of microwave pulses produced with a predetermined pulse repetition rate and having a fixedly predetermined center frequency, which is the same for all measurements. The microwave pulses have, for example, fixedly predetermined center frequencies of 26 GHz or 78 GHz. The antenna is mounted on the container above the highest fill level to be measured, oriented toward the fill substance, and sends the transmission signals into the container. Subsequently, it receives as received signals their signal fractions reflected on reflectors located in the container back toward the fill-level measuring device after a travel time dependent on distance from the respective reflectors. The received signals are fed to a signal processing system connected to the transmission system and the antenna. The signal processing system then determines the fill level based on the received signals.
In such case, measurement curves are regularly derived, which provide the amplitudes of the received signals as a function of their travel time required for the path to the respective reflectors and back. From the travel times of the maxima of these measurement curves, then, based on the propagation velocity of the microwave pulses, the distance from the respective reflectors can be determined by the fill-level measuring device.
For fill level measurement, currently a large number of different evaluation methods are applied. These methods are frequently referred to as echo recognition methods. They are used to ascertain which of the maxima contained in a measurement curve can be attributed to the reflection on the surface of the fill substance. In such case, for example, the first occurring maximum or the maximum having the greatest amplitude is determined to be the maximum of the respective measurement curve attributable to the reflection on the surface of the fill substance. From the travel time of this maximum, based on the propagation velocity of the microwave pulses, the distance of the surface of the fill substance from the fill-level measuring device is derived, which then based on the installed height of the antenna is convertible into the fill level—thus the fill level of the fill substance in the container.
These fill level measuring devices deliver reliable measurement results in a large number of different applications.
For fill level measurement of bulk goods, they are, however, as a rule, not optimally suitable, since bulk goods regularly form hill or valley shaped, bulk goods cones, whose surface profiles cannot be registered with these fill-level measuring devices and, thus, cannot be taken into consideration for fill level determination.
In these applications, consequently, currently clearly more complex, mapping radar measuring systems are applied, with which a surface profile of the surface of the fill substance can be plotted. Mapping radar systems require, however, an antenna system composed of a number of spatially neighbored antennas, which are connected with one another via an electrical control unit, and, depending on the embodiment, serve based on a time schedule as a transmitting antenna, as a receiving antenna or as a transmitting and receiving antenna. In such case, the beam paths of the transmitted and received signals R varied by the electronically controlled activating of different antennas as transmitting, respectively receiving, antennas in such a manner that, based on the signal travel times measured on different beam paths for maxima of the received signals attributable to reflections on the surface of the fill substance in conjunction with the positions of the respectively used transmitting and receiving antennas, a profile of the surface of the fill substance can be calculated.
Alternatively, spatial profiles of fill level upper surfaces can be plotted with fill-level measuring devices, which have a single antenna, which is arranged mechanically swingably over the fill substance. Also in this way, by successively executed measurements with progressively changed antenna orientations, a surface profile of the fill substance upper surface can be ascertained. Mechanically swingable antennas are, however, clearly complex to manufacture, mechanically delicate and maintenance intensive. Another alternative is to use a number of mutually independent, neighboring, fill level measuring devices operated next to one another, whose measurement results are taken into consideration together with their respective positions for determining a surface profile.
Likewise, in given cases, problematic is the use of conventional fill level measuring devices with a single, rigidly mounted antenna in applications, in which installed objects (referred to herein as disturbances), such as e.g. other measuring devices or filling nozzles, are present protruding laterally into the container into the beam path of the transmission signals.
A reliable measurement is then only possible, when the maxima of the measurement curves attributable to the surface of the fill substance can be identified. For this, it is necessary to be able reliably to distinguish the maxima produced by the surface of the fill substance from the maxima of the measurement curves produced by disturbances.
If the position of a disturbance is known beforehand, then the associated travel time range in the measurement curves can, for example, be masked out. Alternatively, measurement curve amplitudes occurring in this region are only taken into consideration, when they exceed a predetermined threshold value. These methods referenced herein as disturbance echo masking methods assume, however, that the position of the disturbance is known beforehand or can be reliably ascertained in some other manner.
Alternatively or supplementally for this, currently so-called-echo tracking methods are applied, in the case of which, in sequential measurements, measurement curves are plotted, and, based on these measurement curves, the time development of maxima of the measurement curves associated with the travel times of certain reflectors, especially the surface of the fill substance, the container floor or the disturbance, is ascertained. Based on this echo tracking, then prognoses for the expected travel times of the maxima of the measurement curves attributable to a certain reflector can be created, which subsequently are applied for improving reliability or for reviewing the associating of the maxima then actually established in the next measurement curve to the respective reflectors. In such case, however, also here, it is necessary that, at the start, at least once, a reliable associating of the maxima to the associated reflectors be predetermined or otherwise ascertained, before their time development can be tracked, respectively their future development predicted.